Spec/Conformance: subtyping callables with non-constant parameter mapping. #2224

Consider the example below, which errors with all tested type checkers (mypy/pyright/ty/pyrefly):

from typing import Protocol

class Interval: ...

class Make(Protocol):
    def __call__(self, /, lower: float, upper: float) -> Interval: ...

def make_impl(
    string_or_lower: str | float | None = None,
    /,
    lower: float | None = None,
    upper: float | None = None,
) -> Interval: ...

def test() -> None:
    _f: Make = make_impl  # ❌️ type checkers error here.

I believe from a pure type theory POV, this assignment should be legal, because all legal arguments to Make are also legal arguments to make_impl. The spec phrases it in the same spirit:

A callable type B is assignable to a callable type A if the return type of B is assignable to the return type of A and the input signature of B accepts all possible combinations of arguments that the input signature of A accepts. All of the specific assignability rules described below derive from this general rule.

And I couldn't find anything else in https://typing.python.org/en/latest/spec/callables.html#assignability-rules-for-callables that would disallow this assignment.

It seems the type-checkers try to match the KEYWORD_OR_POSITIONAL parameters by name, which is incorrect. Make has 3 legal call signatures:

1. Make(float, float)
2. Make(float, upper=float)
3. Make(lower=float, upper=float)

and all these 3 call signatures are supported by make_impl, but the parameter mapping is not constant:

1. make_impl(float, float) -> {lower:string_or_lower, upper:lower}
2. make_impl(float, upper=float) -> {lower:string_or_lower, upper:upper}
3. make_impl(lower=float, upper=float) -> {lower:lower, upper:upper}

So either the spec should demand a constant parameter mapping, or this example should be added to the conformance tests.